Estrogen and its receptors are critical factors in the development, progression, metastasis and treatment of breast cancer. In addition to the classical nuclear estrogen receptors, ERalpha and ERbeta, the 7-transmembrane G protein-coupled estrogen receptor GPER is now recognized as mediating many of the rapid signaling events associated with estrogen action. Anti-estrogen therapies in the form of selective estrogen receptor modulators (SERMs, such as tamoxifen) and selective estrogen receptor down regulators (SERDs, such as Fulvestrant) have been highly successful in many ER-positive breast cancer patients; however, intrinsic and acquired resistance remains significant problems. Tamoxifen and Fulvestrant are agonists of GPER, suggesting that GPER may play a role in resistance to these drugs. We have identified both a selective agonist and antagonists of GPER that will allow us to test the hypothesis that GPER plays an important role in breast carcinogenesis and treatment efficacy. Aim 1 will test whether GPER mediates cellular effects including proliferation, survival, transformation and migration in response to estrogen, anti-estrogens and GPER-selective ligands. Aim 2 will test whether selective targeting of GPER activity modulates carcinogenesis and metastasis in a murine model of breast cancer. Aim 3 will test whether selective activation and inhibition of GPER regulates proliferation and survival of cells in normal human breast tissue and human breast tumor explants. Significance: Completion of these aims will significantly advance our knowledge of and provide insight into the role of the novel estrogen receptor GPER in multiple aspects of breast cancer, from initiation to metastasis and drug resistance resulting in the identification of a novel therapeutic target for which highly selective antagonists exist. Further development of this antagonist could lead to a new drug for the treatment of GPER-expressing breast tumors.